This invention relates to plasma etching or reactive ion etching. More particularly, the invention relates to an improved etchant mixture which does not substantially affect the etch rate on SiO.sub.2 but significantly decreases the etch rate on silicon.
An important step in the manufacture of semiconductor chips and thin film circuitry is the etching of the different layers such as polysilicon and silicon dioxide which make up the finished semiconductor chip or the thin film circuit. In the manufacture of these thin film circuits, one method of etching has been to overlay the surface to be etched with a suitable mask and to immerse the circuit so masked in a chemical solution which attacks the surface to be etched while leaving the mask otherwise intact. It has been difficult with the chemical etching processes presently known to achieve well-defined edges on the etched surfaces. The difficulty arises because the chemical action tends to etch isotropically, i.e., it undercuts the mask by the same distance that it penetrates the underlayer, and thus enlarges the feature. It is, therefore, very difficult to use wet chemical etching to achieve fine structures; fine structures being defined as structures having geometries on the order of one micron.
One known method for manufacturing semiconductor chips is called "plasma etching" in which a vacuum container is filled with a low pressure gas such as a fluorocarbon or SF.sub.6. A surface to be etched is covered by a mask and inserted into the container along with the reactive gas. To etch the surface, an ac voltage whose frequency may be between 20 kHz and 30 Ghz is applied to excite the fluorocarbon or SF.sub.6, thereby dissociating the fluorocarbon or SF.sub.6 and forming various positive and negative ions, reactive neutral species (e.g., F atoms), and electrons. The dissociated species interact with the surface to be etched producing various gases as reaction products.
The ability to etch thin (approximately 1.mu.) layers of SiO.sub.2 over Si is an important aspect of the plasma or reactive ion etch processes used in microcircuit manufacture. The plasma field of any given apparatus or oxide layer thickness is not necessarily uniform over the entire wafer surface, thus the etch may penetrate the SiO.sub.2 layer at different times. In order to avoid etching the underlying silicon underlayer where the oxide has first cleared, it is desirable to arrange a selective plasma process which does not seriously affect or diminish the etch rate on SiO.sub.2 but significantly decreases the rate on silicon.
Considerable prior art exists describing plasma etching wherein various plasma generating compositions, such as CF.sub.4, CHF.sub.3, C.sub.2 F.sub.6, C.sub.2 F.sub.4, CCl.sub.4, BCl.sub.3, etc., including admixtures, with O.sub.2, N.sub.2, Ar or air, are introduced into a reactor under a specific pressure to generate a plasma within the reactor. The material to be etched is introduced into the reactor and the etching process is effected by radicals formed by the plasma. In the past, such compositions have not yielded consistently satisfactory results. Additionally, the use of SF.sub.6 alone or in mixtures with chlorine gas have been employed in selectively etching silicone or polysilicon over silicon dioxide, or where silicon dioxide is used as a mask, as disclosed for example, in U.S. Pat. Nos. 4,052,251, 4,214,946 and 4,330,384. The patentees in U.S. Pat. No. 4,330,384 in column 3, especially at lines 38-41, for example, emphasize this selectivity of SF.sub.6 for silicon over SiO.sub.2. No prior art is known, however, in which the reverse is disclosed, i.e., which describes the use of SF.sub.6 for the selective etching of silicon dioxide preferentially over the underlying silicon.
Accordingly, a need exists for a composition that avoids the drawbacks of the prior art especially a mixture which is efficient, yet does not substantially attack the underlying silicon during the etching process.